The present invention is concerned with adhesive compositions.
Adhesive compositions having prolonged tackiness and bonding properties i.e. so called pressure-sensitive adhesives, are used for a variety of purposes. By the expression "pressure-sensitive adhesive" where used herein is meant an adhesive composition which can be applied to a substrate to provide a layer which has tacky characteristics which are preserved over a wide range of temperatures i.e. at least -20.degree. C. to 50.degree. C. for a period of time in excess of 3 months, and which layer is capable of forming a bond to a surface by the application of light pressure alone. Known uses for pressure-sensitive adhesives include provision of permanently tacky coatings on tapes for various uses, coatings for labels, decorative elements, wall coverings and sheet materials of various types.
Pressure-sensitive adhesive compositions are formulated to provide compositions having a certain minimum balance of physical and chemical properties so as to be able to withstand the stresses and environment that the product will encounter and be expected to survive in use. The physical properties known in the art as tack, peel adhesion, and shear adhesion (otherwise referred to as holding power) are important in determining the suitability of any given pressure-sensitive adhesive composition to provide the necessary application properties and in-service durability required of the pressure-sensitive adhesive product.
The first property, tack, is in effect the instantaneous adhesion of the adhesive layer to a surface of an object that is developed immediately after contacting the surface with the layer. One accepted method of measuring tack, endorsed by the Pressure Sensitive Tape Council (USA), is test method PSTC-6 evolved and published as a standard test by the Council. It involves rolling a stainless steel ball down an inclined plane onto the surface of a pressure-sensitive adhesive layer deposited on a horizontal plane, and measuring the distance the ball rolls across the adhesive. In this test, tack is expressed in distance of ball travel. Thus high tack adhesive stops the ball within 0.5 to 3 inches (about 1.27 to 7.6 cm), whereas low tack adhesives allow the ball to roll more than 12 inches (about 30.3 cm).
The second property, peel adhesion, is the adhesion of the pressure-sensitive adhesive product after the adhesive layer has been in contact with an adherent surface for a sufficient period of time to reach a steady state or equilibrium with the surface. It is normally measured as the force required to remove a pressure-sensitive tape from a panel or its own backing at a specified angle and speed. PSTC-1 specifies one standard test method.
The third physical property utilised to define the suitability of a pressure-sensitive adhesive product for a given use is its shear adhesion. This test measures in essence the durability and permanence of the adhesive bond to a surface when subjected to a constant force, in the direction of the surface. PSTC-7 specifies one standard test method.
Besides these physical property parameters, the chemical attributes of the pressure-sensitive adhesive composition must also be considered in order to provide a pressure-sensitive adhesive product having the requisite durability in the environment in which it is to be used. Thus, for example, the pressure-sensitive adhesive must be formulated so as to exhibit resistance to adhesive-degrading environmental conditions such as elevated temperatures, sunlight, organic solvents, moisture, acids, and bases to which the pressure-sensitive adhesive product will be exposed in service.
Various polymeric materials have been used as a basis for pressure-sensitive adhesives, including natural rubber, styrene butadiene copolymers, acrylate copolymers and polyurethanes. Generally speaking the acrylate copolymers are the most frequently preferred materials for pressure-sensitive adhesives in view of their excellent ageing resistance, light stability, adhesion to a wide range of substrates, and due to the provision of a variety of acrylate copolymers to give various blends of tack, cohesion and adhesive properties.
Whilst acrylate copolymer based pressure-sensitive adhesives are available in a variety of forms (i.e. as solutions, emulsions and even hot melts) having a variety of properties, the use of those adhesives which are required to cure after application through use of curing agents or require use of quantities of energy to dry or cure them after application tends to be comparatively expensive. Also, those materials intended for application as hot melts tend to have comparatively low heat resistance properties.
Various proposals have been made to employ polyurethanes as the polymer basis for pressure-sensitive adhesives, but frequently these have employed two-component systems which require mixing before application, or blocked isocyanates which require significant heating to liberate the isocyanate, or less commonly available raw materials.
U.S. Pat. No. 3,437,622 discloses polyurethane based pressure-sensitive adhesives made from commonly available urethane raw materials, and optionally tackifying resin or plasticiser. The polyurethanes disclosed are made from aromatic diisocyanates and diols of hydroxyl number between about 56 and about 265 and triols of hydroxyl number between about 28 and about 56. The specification describes the use of these materials as solutions in organic solvent, and the use of heated circulating air ovens heated to temperatures of 100.degree. C. or more for completion of the formation of the desired polyurethane and removal of the solvent. However, the ability of adhesives taught in U.S. Pat. No. 3,437,622 to provide a desired balance of tack, peel adhesion, creep resistance and environmental resistance has been questioned, and further formulations proposed. See for example U.S. Pat. Nos. 3,846,163 and 3,879,248.
U.S. Pat. No. 3,681,277 discloses a process for preparing solvent free pressure-sensitive adhesive from mixtures of polyisocyanate and polyoxyalkylated diol or polyol and a tackifier. This specification discloses that the polyol may be a poly(alkylene oxide) which is unbranched, branch-chained or partly branched. Examples given of suitable compounds are those obtained by the polyoxyalkylation of dihydric or polyhydric, preferably dihydric and/or trihydric, alcohols, such as ethylene glycol, propylene glycol, trimethylol propane and glycerol, with ethylene oxide and/or preferably propylene oxide. Suitable poly(alkylene oxides) have molecular weights of from 300 to 10,000 for example from 900 to 5,000 and preferably from 1,500 to 4,000 and more preferably from 2,100 to 3,000 and hydroxyl numbers ranging from 30 to 85, preferably from 45 to 75 and more preferably from 50 to 65. The specification is directed primarily at producing OH bearing end products, and requires mixing of the components immediately prior to use i.e. the mixture is not storage stable.
It would be economically beneficial to users of pressure-sensitive adhesives to have available a pressure-sensitive adhesive capable of simple application to provide a high quality pressure sensitive adhesive without the need to mix materials immediately prior to application, or the necessity to provide heating or drying equipment. It would also be beneficial to provide a pressure-sensitive adhesive based on commonly available, inexpensive, raw materials which can be readily applied at room or slightly elevated temperature to required substrates and then capable of becoming cured under relatively mild conditions and so achieve a desirable combination of pressure-sensitive and other properties.
It is one of the objects of the present invention to provide an improved pressure-sensitive adhesive.